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USB ground Isolation
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mosc
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PostPosted: Tue Feb 06, 2007 12:30 pm    Post subject: USB ground Isolation
Subject description: Ground loop noise reduction
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There are several topics on the electro-music.com Nord Modular sub-forums about noise (often high-pitched) leaking through when audio equipment is connected to computers via USB. Most of the time these problems are solved by removing the ground connection from the computer to the AC mains by using ground lifting plug or just breaking of the ground pin on the end of the computers power cord.

In cases where this is not enough, it is possible to try using an expensive higher quailty USB cord. There have been some questions about cutting the ground connection in a USB cable to reduce such ground loop noise. I have done a little bit of internet research about this. For reference see this http://www.usb.org/developers/docs/ .

The USB data is communicated over two wires. The cables contain 4 wires. The signal is on pins 2 and 3. The other two are for a DC power source, V+ and Gnd. This is used to power small remote devices. If a USB device, like a synthesizer, doesn't get it's power from the USB cable, then it should be perfectly safe to cut these wires, and thus remove a potential source of unwanted noise.

The data is transmitted on a differential pair which doesn't require a ground, or in many cases it doesn't even require shielding. In the case of a differential signal, the ground can do more harm than good.

In cases where there is no other solution to solving your noise problems, try modifying your USB cables by cutting the V+ and GND wires.

Here is the pinout on the USB cable.

Posted Image, might have been reduced in size. Click Image to view fullscreen. Posted Image, might have been reduced in size. Click Image to view fullscreen.

Pin
one side
Signal Description Pin
other side
1USB Vcc (Vbus) usually RED, wire should be 20-28 AWG1
2USB Data - usually WHITE, wire should be 28 AWG2
3USB Data usually GREEN, wire should be 28 AWG3
4GND usually BLACK, wire should be 20-28 AWG4

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Blue Hell
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PostPosted: Tue Feb 06, 2007 1:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

There is one extra connection that was not mentioned in the above and it might be the worst one regarding the problem. The shielding ground is connected on both sides of the cable on some I neasured (don't know what the specs say, will try to find out).

I'd first try to remove that particualr connection before tampering with the power leads. The GND wire (not the shielding ground) might be needed for detection of connection/disconnection - must check the specs here as well to be sure.

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PostPosted: Tue Feb 06, 2007 1:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes, please check the specs and set me straight if I'm wrong about something.
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PostPosted: Tue Feb 06, 2007 2:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

The USB speed detection works with a pullup resistor in the client or the function as it's called (i.e. not the computer but the other end), the voltage it is pulled up with should be derrived from the voltage supplied over the USB bus. So leaving out the power wires will make the speed detection impossible and the bus might not boot, or maybe it will run low speed only.

Re. the shielding ground connections the spec is not quite clear, the following short section is all I could find about it.

Quote:
6.8 USB Grounding
The shield must be terminated to the connector plug for completed assemblies. The shield and chassis are
bonded together. The user selected grounding scheme for USB devices, and cables must be consistent with
accepted industry practices and regulatory agency standards for safety and EMI/ESD/RFI.


Which to me seems to mean that conecting the shield at both sides of a cable is the right thing to do, and it also seems to mean that it is not specified which device should not connect the shield to the GND. This is odd. I could not find one single diagram in the specs which would indicate how the shield should be handled.

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PostPosted: Tue Feb 06, 2007 2:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

There is a USB 2.0 white paper out there somewhere with an extensive take on the cabling/connnectors. I used that PDF when I messed with a similar issue a year ago. I think I found it at some tech org or engineering site.
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PostPosted: Tue Feb 06, 2007 2:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

There are of course various products out there that offer galvanic isolation and that jazz.

This is one of them.
http://www.crupower.fi/usbgt.pdf

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PostPosted: Tue Feb 06, 2007 3:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

FWIW, for the G2 the shield is not connected at the device (function) side.
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PostPosted: Tue Feb 06, 2007 3:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Aha! At least now we know that.
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PostPosted: Tue Feb 06, 2007 3:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

come on guys, how come you keep writing about the same subject on two different threads Question my eyes hurt Wink
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PostPosted: Tue Feb 06, 2007 3:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

It's not the same subject, this one is general, the other two are G2 specific.
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PostPosted: Tue Feb 06, 2007 3:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
It's not the same subject, this one is general, the other two are G2 specific.

a total of three threads on USB ground isolation Question Shocked
btw I used to connect my Kawai MP4 to the Powerbook thru USB getting a little but annoying buzz. when I switched back to connection thru a midi/usb interface the noize disapperead.

Last edited by seraph on Tue Feb 06, 2007 3:44 pm; edited 1 time in total
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PostPosted: Tue Feb 06, 2007 3:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

seraph wrote:



a total of three threads on USB ground isolation Question Shocked



Sure, according to Dr. Ruth, three times is good for you.

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